In recent years, electronic devices have rapidly been converted to portable and wireless. As a driving source for such devices, a high voltage nonaqueous electrolyte secondary battery having high energy density is coming into practical use. Not only to such small-size consumer products, the technology of the nonaqueous electrolyte secondary battery is also applied to a high capacity battery used for energy storages and electric vehicles and to a high power battery used for electric tools and hybrid electric vehicles (HEV). The nonaqueous electrolyte secondary battery includes an electrode group prepared by winding strip-like positive and negative electrodes, each of which is made of an electrode material mixture layer and a current collector, together with a separator which electrically insulates the positive and negative electrodes and supports an electrolyte thereon. In general, a thin microporous polyethylene sheet having a thickness of several tens μm is used as the separator.
For increasing the power of the nonaqueous electrolyte secondary battery, the electrode group of the secondary battery is configured into a so-called “tabless” electrode group. To be more specific, each of the positive and negative current collectors carrying an active material thereon is prepared such that an end of the current collector is exposed. Then, the exposed end of the current collector is welded to a current collector terminal. Accordingly, a current path to every part of the strip-like electrode is uniformly ensured and the output property is improved.
If the tabless electrode group is adopted, the exposed part of the current collector and the current collector terminal are welded near the separator. In general, the separator is made of a resin such as polyethylene and therefore less heat resistive. Therefore, the separator may partially melt in the welding process and an internal short circuit may occur between the positive and negative electrodes. Even if the internal short circuit does not occur, the separator may shrink as it partially melts. As a result, part of the current collector is deformed to bring about buckling of the electrode group.
As a measure to prevent the melting of the separator and the insulation failure between the electrodes, Patent Literature 1 describes a method of keeping the tip of the separator and the tip of the active material layer formed on the current collector at a certain distance from the tip of the exposed part of the current collector in the process of welding the exposed part of the current collector and the current collector terminal.    Patent Literature 1 Japanese Unexamined Patent Publication No. 2004-253253    Patent Literature 2 Japanese Unexamined Patent Publication No. 2000-100408    Patent Literature 3 Japanese Unexamined Patent Publication No. 2006-351386